ST. PETERSBURG, FL 鈥� Congratulations to Dr. Pamela Hallock Muller on now judging her 鈥淣utrient excess and the demise of coral reefs and carbonate platforms鈥� paper as a landmark in the category Carbonate Sedimentology and Stratigraphy.
Abstract
Growth rates of corals on Holocene reefs indicate that carbonate platforms should
easily keep pace with long-term subsidence and sea-level changes, yet drowned reefs
and platforms are common in the geologic record. Recognition of the negative influence
of nutrients on reef communities provides a clue to that paradox. The primary carbonate-sediment
producers of the coral reef community are highly adapted to nutrient-deficient environments.
Input of nitrates and phosphates stimulates growth of plankton, which reduces water
transparency, limiting depth ranges of zooxanthellate corals and calcareous algae
and thereby reducing carbonate production. Higher nutrient concentrations and plankton
densities also stimulate growth of fleshy algae and ahermatypic suspension-feeding
animals in the benthos. Besides displacing hermatypic algae and corals, many of these
fast-growing competitors are bioeroders that actively destroy the reefal structure.
Because rates of carbonate production and bioerosion are similar, even modest increases
in nutrient availability can shift a reef community from net production to net erosion.
In the geologic record, drowned reefs and carbonate platforms typically exhibit evidence
of nondeposition, bioerosion, and reduced redox potential, which indicate excess nutrient
availability during drowning. Drowned reefs overlain by shales are possible victims
of nutrients in terrestrial runoff that suppressed reef growth before arrival of siliciclastic
sediments. Other drowned platforms may have succumbed during rapid pulses of sea-level
rise that flooded previously subaerial platforms. Nutrients in the soils of the flooding
platform were mixed into surface waters, suppressing reef growth. The reef drowned
if submergence proceeded beyond the critical depth before the excess nutrients were
exported from the system. Other mechanisms for reef drowning by excess nutrients include
changes in local or regional upwelling patterns or mid-ocean overturn.